The present invention relates in general to a downhole apparatus and method for generating electricity and, in particular to, a downhole electrical generator that uses lift fluid pressure to produce electricity which is used to operate other downhole devices.
Without limiting the scope of the invention, its background is described in connection with the operation of downhole electrical devices, as an example. The control and operation of oil and gas production wells constitute an important and ongoing concern of the petroleum industry. As an example, well control has become particularly important and more complex in view of the industry wide development of multilateral wells. Generally speaking, multilateral wells have multiple branches each having discrete production zones which produce fluid into common or independent production tubing. In either case, there is a need for controlling zone production, isolating specific zones and otherwise monitoring each zone in a particular well. As a result, the methods and devices used for controlling wells are growing more complex. In fact, downhole control systems which include downhole computerized modules employing downhole computers for commanding downhole tools such as packers, sliding sleeves and valves are becoming more common.
For example, using downhole sensors, a downhole computer controlled system may monitor actual downhole parameters such as pressure, temperature and flow to automatically execute control instructions based upon the monitored downhole parameters. As should apparent, operating such a well control systems will require electrical power. It has been found, however, that presently known methods of supplying or generating electricity downhole suffer from a variety of problems and deficiencies.
In one method, electricity may be supplied downhole by lowering a tool on a wireline and conducting electricity through one or more conductors in the wireline from the surface to the tool. Similarly, hardwires may be attached on the exterior of the tubing running from the surface to the desired downhole location. These techniques, however, are not desirable due to their cost and complexity. In addition, in deep wells, there can be significant energy loss caused by the resistance or impedance in the wires.
Downhole electrical circuits utilizing batteries housed within a downhole assembly have also been attempted. These batteries, however, can only provide moderate amounts of electrical energy at the elevated temperatures encountered downhole. In addition, batteries have relatively short lives requiring frequent replacement and/or recharging.
Other attempts have been made to provide a downhole mechanism which continuously generates and supplies electricity. For example, systems using radioisotopes, fuel cells and piezoelectric techniques have been attempted. These systems, however, have raised safety and environmental concerns, are expensive and complex and/or do not generate suitable amounts of electricity.
A more promising approach to supplying electricity downhole appears to be the use of downhole electrical generators. Previous attempts to operate downhole generators, however, have met with limited success. Specifically, many downhole generators are installed within the tubing string which prevents the passage of other tools or equipment therethrough. Other downhole generators have been proposed that are installed in side pockets thus allowing passage of equipment through the tubing.
All of these downhole generators, however, suffer from a serious drive problem. Specifically, the turbines of these downhole generators are rotated by the upward flow of production fluids. Not only does this create an undesirable pressure drop in the production fluids, but use of production fluids to drive turbines significantly limits the life expectancy of these downhole generators. Specifically, the mechanical and chemical qualities of production fluids tend to erode and corrode the turbine as well as other components of these downhole generators. In addition, tars and suspended solids in the production fluid tend to clog flow passageways within these downhole generators and prevent proper rotation of the rotors. Also, the amount of the electrical output of these production fluid driven downhole generators is controlled by the flow rate of production fluid through the tubing which is dependent, in part, upon the pressure in the formation which decreases over time.
Therefore, a need has arisen for a downhole generator that is not driven by the flow of production fluids through the tubing. A need has also arisen for such a downhole generator that does not cause a pressure drop within the production fluids. Further, a need has arisen for such a downhole generator wherein the electrical output is not dependent upon the pressure in the formation from which the production fluids are produced.
The present invention disclosed herein comprises a lift fluid driven downhole electrical generator that does not use the flow of formation fluids to drive a turbine. As such, the lift fluid driven downhole electrical generator of the present invention does not choke the flow of formation fluids up through the tubing. In addition, the electrical output of the lift fluid driven downhole electrical generator of the present invention is not dependent upon the flow rate of formation fluids or the pressure in the formation from which the formation fluids are produced.
Broadly characterized, the lift fluid driven downhole electrical generator, once positioned downhole in a tubing string, converts the lift fluid pressure into electricity. For example, the lift fluid may be used to create rotary motion by impinging the lift fluid against a rotor. The rotary motion may then be converted to electricity by rotating a first portion of an electromagnetic assembly relative to a second portion of the electromagnetic assembly.
The lift fluid driven downhole electrical generator comprises a housing having one or more lift fluid ports in a sidewall portion thereof for receiving the lift fluid from the annulus surrounding the tubing string. A flow control device that is slidably disposed within the housing is used to selectively allow and prevent the flow of lift fluid through the lift fluid port. The openness of the lift fluid port may be controlled by the operation of an actuator that is operably coupled to the flow control device. The actuator may infinitely vary the openness of the lift fluid port between the fully open and fully closed positions in response to a signal from the surface received by a downhole telemetry system, a signal from a downhole sensor or a timer. Alternatively, a controller may be used to monitor the electrical output of the downhole generator and then send a signal to adjust the position of the flow control device relative to the lift fluid port to vary the electrical output of the downhole generator if desired.
When the lift fluid ports are open, a rotor, rotatably disposed within the housing, converts the lift fluid pressure to rotary motion as the lift fluid impinges the rotor. The rotation of the rotor is imparted on the first portion of the electromagnetic assembly which is rotatable relative to the second portion of the electromagnetic assembly, which is stationary with the housing. This relative rotation within the electromagnetic assembly converts the rotary motion to electricity. The first portion of the electromagnetic assembly includes a plurality of electrical windings wrapped around a core. One end of the electrical windings is electrically coupling to a first portion of a commutator and the other end of the electrical windings is electrically coupling to a second portion of the commutator. The second portion of the electromagnetic assembly includes magnets and at least two contact members that are stationary with the housing of the downhole electrical generator. In operation, when the first portion of the electromagnetic assembly is rotated relative to the second portion of the electromagnetic assembly, a first contact member sequentially engages the first portion of the commutator then the second portion of the commutator while a second contact member simultaneously sequentially engages the second portion of the commutator then the first portion of the commutator. As such, electricity is generated by the lift fluid driven downhole electrical generator of the present invention.
In addition, the present invention may be used to control the electrical output of a lift fluid driven downhole electrical generator. This is achieved by positioning the downhole electrical generator within a tubing string, injecting a lift fluid down an annulus surrounding the tubing string, providing a fluid communication path through the downhole electrical generator by varying the position of a flow control device relative to a lift fluid port, communicating lift fluid through the lift fluid port, rotating a rotor and an electromagnetic assembly such that electricity is generated in response to the flow of lift fluid through the fluid communication path, sensing the generated electricity to determine the electrical output of the downhole electrical generator and adjusting the flowrate of lift fluid through the fluid communication path by selectively varying the position of the flow control device relative to the lift fluid port, thereby controlling the electrical output of the downhole generator.
More specifically, the step of sensing the generated electricity to determine the electrical output of the downhole electrical generator may include receiving a signal indicative of the magnitude of the electricity being generated with a controller, processing the signal in the controller and generating a control signal with the controller to vary the position of the flow control device relative to the lift fluid port.